The present invention generally relates to an image processing apparatus, and more particularly to an image processing apparatus, which is applicable to image forming systems such as a laser printer, a digital copier and a facsimile machine, for eliminating distortions from an image so that an image with smooth edges is produced.
In image forming systems such as dot matrix printers and display devices, a line image such as a character which is described by lines is converted into a dot image using font data, and a dot image such as a photograph which is described by dots is read by an image scanner from a original image. These images are quantized or digitized into binary image data, and stored, for example, in a video data area of a memory (e.g., a random access memory RAM) in the image forming system. A bit mapped image is thus produced in the memory as the result of the quantization. The bit mapped image is read out from the memory, and this image is supplied to an image forming part or a displaying device, so that the image is printed on a recording sheet, or displayed on a CRT (cathode ray tube) screen.
When an analog image is formed by an image forming system, the elements of the image are continuous at any location in all directions. However, when a digital image expressed in binary data after quantization, or a bit mapped image described by dots arranged in columns and lines (dot matrix formation) is formed, it is difficult to produce a continuous image from the bit mapped image. Generally, distortions are produced at edges of the image when a digital image is output (printed or displayed). Also, when an image of an inclined line having a gradient at a certain angle to the horizontal or vertical direction is output, the image edges appear jagged. It is difficult to produce a desired continuous image which is the same as the original image with no distortion.
One conceivable method for effectively eliminating distortions from the output image is to increase dot density of the output image by reducing the size of each dot of the bit mapped image so that a high resolution is obtained for the output image. However, in order to obtain a high resolution for the output image, it is necessary to increase remarkably the cost of the image forming system needed to apply such a method to the image forming. For example, in order to increase the resolution of a bit mapped image described by 300.times.300 dots per inch (dpi) to a 600.times.600 dpi resolution which is twice as high as the original resolution, it is necessary that the capacity of the memory and the speed of data processing are made four times higher than those of the original image forming system.
Another method for eliminating the distortion from the output image is to use an interpolation technique. By using the interpolation technique, an input image with staircase-like irregular edges is converted into an output image with a continuous slope. Or, by averaging intensities of adjacent dots of an input image, such irregular edges of the image are corrected to an output image with edges having smoothly changing intensities. However, when the interpolation is used, fine details are eliminated from the input image, and there is a problem in that the output image has a poor contrast and a low resolution.
In order to resolve the above mentioned problems, there has been proposed a smoothing device for smoothing edges of a displayed character. U.S. Pat. No. 4,544,922 discloses a smoothing device for a display apparatus. In the smoothing device disclosed in this patent, a display image of a character composed of selected standard width dots of a matrix of orthogonally disposed rows and columns is smoothed in response to data stored in a memory. The smoothing involves the selective addition of small dots having a predetermined width to particular portions of the character, and the selective removal of such dots. In the disclosed device, conventional pattern recognition or template matching is performed, in order to detect the particular portions of the character to be smoothed.
However, in the above smoothing device, pattern recognition or template matching is performed for each position of dots of a bit mapped image so that the small dot is added to or removed from the detected portions of the character in accordance with the result of the pattern recognition or template matching. Therefore, the above smoothing device is very expensive, and requires a long processing time for obtaining a smoothed character image with a high picture quality.
Moreover, in order to eliminate the above described problem, there has been proposed an image forming apparatus. Japanese Laid-Open Patent Publication No. 2-112966 discloses this type of an image forming apparatus. In the image forming apparatus of this publication, template matching is performed by comparing each subset of dots (including a target dot and neighboring dots thereof) of an input image (a bit mapped image) with one of predetermined patterns of dots which are stored in a memory as templates. The predetermined patterns of dots of each template include specified features of a bit mapped image for each input image. If one subset of dots of the input image including a target dot matches with one of the predetermined patterns, the subset of dots of the input image is replaced by a subset of compensation dots corresponding to the pattern with which the subset has matched, in order to improve the quality of the output image.
However, in order to accomplish the image forming with improved quality when the above mentioned image processing apparatus is used, it is necessary that a large number of predetermined patterns of dots (or templates) including specified features of a bit mapped image for any input image is prepared and stored in the memory. The memory must have a very great capacity to store the large number of predetermined patterns of dots. The time and labor needed to prepare and store such dot patterns are considerable.
It is also necessary that the template matching to detect specified features of a bit mapped image is performed with respect to each target dot of the input image. Thus, when the above described image forming apparatus is used, the template matching is time consuming.